Magnetic particle imaging in vascular medicine

Abstract

Magnetic particle imaging (MPI) is a new medical imaging technique that enables three-dimensional real-time imaging of a magnetic tracer material. Although it is not yet in clinical use, it is highly promising, especially for vascular and interventional imaging. The advantages of MPI are that no ionizing radiation is necessary, its high sensitivity enables the detection of very small amounts of the tracer material, and its high temporal resolution enables real-time imaging, which makes MPI suitable as an interventional imaging technique. As MPI is a tracer-based imaging technique, functional imaging is possible by attaching specific molecules to the tracer material. In the first part of this article, the basic principle of MPI will be explained and a short overview of the principles of the generation and spatial encoding of the tracer signal will be given. After this, the used tracer materials as well as their behavior in MPI will be introduced. A subsequent presentation of selected scanner topologies will show the current state of research and the limitations researchers are facing on the way from preclinical toward human-sized scanners. Furthermore, it will be briefly shown how to reconstruct an image from the tracer materials' signal. In the last part, a variety of possible future clinical applications will be presented with an emphasis on vascular imaging, such as the use of MPI during cardiovascular interventions by visualizing the instruments. Investigations will be discussed, which show the feasibility to quantify the degree of stenosis and diagnose strokes and traumatic brain injuries as well as cerebral or gastrointestinal bleeding with MPI. As MPI is not only suitable for vascular medicine but also offers a broad range of other possible applications, a selection of those will be briefly presented at the end of the article.

Keywords: MPI scanner; cardiovascular intervention; functional imaging; image reconstruction; magnetic nanoparticles; medical imaging; quantitative imaging; real-time imaging.

Figure 1:

The signal encoding principle in MPI is based on the excitation of MNPs by an oscillating magnetic field called drive field H^D(t). Based on a modulation of the drive field by the particles’ non-linear magnetization curve, a characteristic receive signal u(t) can be induced. As shown by the frequency spectrum û(t) of such a voltage signal, the modulation of H^D(t) causes the generation of higher harmonics of the applied frequency. The principle for spatial encoding in MPI is based on the application of a selection field H^S(i). All MNPs outside the area of the FFP or FFL or rather in a close vicinity to it remain in saturation, and thus do not contribute to the particle signal.

Figure 2:

By use of a continuous data acquisition, different trajectories can be applied to cover the FOV. For a 2D or even 3D imaging approach, a sequential movement (A) or a Lissajous trajectory (B) can be used.

Figure 3:

Schematic drawings of MPI scanners in bore and single sided geometry. (A) Schematics of the principal scanner setup developed by Philips (according to Ref. [1]). (B) The principle coil geometry of a single-sided MPI scanner. It consists of two concentrically aligned transmit coils and a dedicated receive coil. For 3D imaging, additional D-shaped transmit and receive coils need to be added (according to Refs. [20], [21]).

Figure 4:

Principle concept of the system matrix acquisition: a tracer sample with known volume and tracer concentration is placed sequentially at every position in the FOV. For every sample position, the corresponding receive signal is acquired and stored in frequency space as one entry in the system matrix.

Figure 5:

In vivo measurements of a beating mouse heart. Resovist was injected into the tail vein of the mouse. The scanner achieves a FOV of 20.4×12×16.8 mm³ and a temporal resolution of 21.5 ms. The MPI images are fused with static MRI images (with permission taken from Ref. [9]). (A) Bolus reaches field of view through vena cava. (B) Tracer reaches right artrium. (C) Bolus centre in right ventricle. (D) Tracer circulated through the pulmonary vessel. (E) Tracer starts filling left ventricle.

Figure 6:

The axial, sagittal, and coronal images (left to right) show a balloon catheter labeled with MNPs (top) and a non-labeled balloon catheter in a vessel phantom filled with MNPs (bottom). The FOV was 20×36×36 mm³ (with permission taken from Ref. [37]).

Figure 7:

Color coding can be used to differentiate between a varnished catheter (colored) and a filled lumen (gray scale). This is possible because the physical properties of MNPs in varnish are different from the properties of MNPs in a fluid (with permission taken from Ref. [46]).

Figure 8:

Sagittal (top) and axial (bottom) slices extracted from reconstructed 3D image data of the stenosis phantoms. The lumen of the 1 mm stenosis is not discernible (with permission taken from Ref. [52]).